Articles including expanded polytetrafluoroethylene membranes with serpentine fibrils having a discontinuous fluoropolymer layer thereon

ABSTRACT

Articles comprising an expanded polytetrafluoroethylene membrane having serpentine fibrils and having a discontinuous coating of a fluoropolymer thereon are provided. The fluoropolymer may be located at least partially in the pores of the expanded fluoropolymer membrane. In exemplary embodiments, the fluoropolymer is fluorinated ethylene propylene. The application of a tensile force at least partially straightens the serpentine fibrils, thereby elongating the article. The expanded polytetrafluoroethylene membrane may include a microstructure of substantially only fibrils. The articles can be elongated to a predetermined point at which further elongation is inhibited by a dramatic increase in stiffness. In one embodiment, the articles are used to form a covered stent device that requires little force to distend in the radial direction to a first diameter but is highly resistant to further distension to a second diameter (stop point). A large increase in diameter can advantageously be achieved prior to reaching the stop point.

FIELD OF THE INVENTION

The present invention relates to expanded polytetrafluoroethylene(ePTFE) membranes having serpentine fibrils and a discontinuousfluoropolymer layer and to materials made therefrom.

Definitions

As used herein, the term “serpentine fibrils” means multiple fibrilsthat curve or turn one way then another.

As used herein, the term “controlled retraction” refers to causingarticles to shorten in length in at least one direction by theapplication of heat, by wetting with a solvent, or by any other suitablemeans or combinations thereof in such a way as to inhibit folding,pleating, or wrinkling of the subsequent article visible to the nakedeye.

The term “imbibed or imbibing” as used herein is meant to describe anymeans for at least partially filling at least a portion of the pores ofa porous material such as ePTFE or the like.

The term “elongation” as used herein is meant to denote the increase inlength in response to the application of a tensile force.

The term “discontinuously located” as used herein refers to a substancehaving at least one unconnected region.

The term “precursor membrane” as used herein refers to the startingmembrane.

The terms “stent graft” and “covered stent” may be used interchangeablyherein to describe a stent with a cover thereon.

The term “increase in stiffness” as used herein refers the increase inresistance to further elongation once the stop-point is reached.

For purposes of this invention, the entire device is considered to be“wrinkle-free” if within a 1 cm length of the device, the graft portionis devoid of wrinkles and folds. It is to be noted that the terms “freeof folds”, “devoid of folds”, and “fold free” are used interchangeablyherein.

BACKGROUND OF THE INVENTION

Porous fluoropolymer materials, and in particular, expandedpolytetrafluoroethylene (ePTFE) materials, typically exhibit relativelylow elongation when stressed in the direction parallel to theorientation of the fibrils. High strength ePTFE materials haverelatively low elongation values compared to lower strength ePTFEmaterials. Uniaxially expanded materials can exhibit high elongationwhen stressed in the direction orthogonal to the fibrils; however, themembranes are exceptionally weak in this direction.

Uniaxially expanded ePTFE tubes positioned on mandrels have beenmechanically compressed and heat treated to achieve higher elongationsprior to rupture. Such tubes also exhibit recovery if elongated prior torupture and released from stress. U.S. Pat. No. 4,877,661 to House, etal. discloses porous PTFE having the property of rapid recovery and amethod for producing these materials. Additionally, the pores ofcompressed tubes have been penetrated with elastomeric materials. Forexample, U.S. Pat. No. 7,789,908 to Sowinski, et al. discloses anelastomeric recoverable PTFE material that includes longitudinallycompressed fibrils of an ePTFE material penetrated by an elastomericmaterial within the pores which define an elastomeric matrix.

A need continues to exist for thin, strong membranes that exhibit highdegrees of elongation, such as greater than 50% elongation. Someapplications further demand qualities such as thinness, low density,and/or small pore size, as well as combinations thereof. Otherapplications require a relatively low force to elongate the membrane.

SUMMARY OF THE INVENTION

The present invention is directed to fluoropolymer membranes thatexhibit high elongation while substantially retaining the strengthproperties of the fluoropolymer membrane. Such membranescharacteristically possess serpentine fibrils having a width of about1.0 micron or less.

It is an object of the present invention to provide an article thatincludes an expanded fluoropolymer membrane having a discontinuouscoating of a fluoropolymer thereon. The fluoropolymer may be located atleast partially in some or all of the pores of the expandedfluoropolymer membrane. The expanded fluoropolymer membranecharacteristically contains serpentine fibrils, and may have amicrostructure of substantially only serpentine fibrils. The serpentinefibrils have a width less than about 1.0 micron or less. In oneexemplary embodiment, the expanded fluoropolymer membrane includes aplurality of serpentine fibrils. In at least one embodiment of theinvention, the fluoropolymer membrane is expandedpolytetrafluoroethylene. One exemplary fluoropolymer is fluorinatedethylene propylene. The application of a tensile force at leastpartially straightens the serpentine fibrils, thereby elongating thearticle. The composite material exhibits high elongation whilesubstantially retaining the strength properties of the fluoropolymermembrane. Additionally, the expanded fluoropolymer membrane may bethermally retracted in at least one direction to less than about 90% ofthe initial, expanded fluoropolymer length. Also, the expandedfluoropolymer membrane may be restrained in at least one directionduring the thermal retraction.

It is another object of the present invention to provide anendoprosthetic device that includes a tubular member defining at leastone opening, an interior surface, and an exterior surface where thetubular member includes a composite material including a fluoropolymermembrane having serpentine fibrils and a discontinuous coating of afluoropolymer. The serpentine fibrils have a width less than about 1.0micron or less. The fluoropolymer may be located at least partially insome or all of the pores of the expanded fluoropolymer membrane. In oneor more exemplary embodiment, the fluoropolymer membrane includesexpanded polytetrafluoroethylene and the fluoropolymer includesfluorinated ethylene propylene. The composite material exhibits anincrease in stiffness when expanded to a diameter of about 7 mm. Thetubular member may be used as a covering for a stent.

It is a further object of the present invention to provide a stent graftthat includes (1) a stent having a wall with at least one opening, anexterior surface, and an interior surface and (2) a cover affixed to thestent where the cover includes a composite material including anexpanded polytetrafluoroethylene membrane having serpentine fibrils anda discontinuous coating of a fluoropolymer. The fluoropolymer may belocated at least partially in all or substantially all of the pores ofthe expanded fluoropolymer membrane. The serpentine fibrils have a widthof about 1.0 micron or less. The composite material at least partiallycovers at least one of the interior and exterior surfaces of the stent.In addition, the composite material may be affixed to the exteriorand/or interior surface of the stent. The fluoropolymer may befluorinated ethylene propylene. The composite material remainswrinkle-free and fold-free regardless of the diameter of the stentgraft. Also, the composite material exhibits high elongation whilesubstantially retaining the strength properties of thepolytetrafluoroethylene membrane. The expanded polytetrafluoroethylenemembrane may include a microstructure of substantially only serpentinefibrils. In one embodiment, the expanded fluoropolymer membrane mayinclude a plurality of serpentine fibrils. The composite materialexhibits an increase in stiffness when expanded to a diameter of about 7mm.

It is also an object of the present invention to provide a stent grafthaving (1) a wall with at least one opening, an exterior surface, and aninterior surface and (2) a cover affixed to the stent where the coverincludes a composite material including an expanded fluoropolymermembrane and a discontinuous coating of a fluoropolymer thereon. Thecomposite material exhibits an increase in stiffness when expanded to adiameter of about 7 mm. Additionally, the composite material at leastpartially covers at least one of the interior and exterior surfaces ofthe stent. It is to be appreciated that the fluoropolymer is notsubstantially adhered to the stent. The fluoropolymer may be fluorinatedethylene propylene. The expanded polytetrafluoroethylene membrane mayinclude a microstructure of substantially only serpentine fibrils. In atleast one exemplary embodiment, the expanded fluoropolymer membrane mayinclude a plurality of serpentine fibrils.

It is yet another object of the present invention to provide a method offorming a covered stent that includes (1) positioning a first tubularmember on an interior surface of a stent, (2) positioning a secondtubular member on an external surface of the stent, where each tubularmember includes a composite material having an expandedpolytetrafluoroethylene membrane and a discontinuous coating of afluoropolymer thereon and the expanded polytetrafluoroethylene membraneincludes serpentine fibrils, and (3) heating the stent having thereonthe first and second tubular members to adhere the fluoropolymer on thefirst tubular member to the second tubular member through, intersticesof the stent and form a covered stent. The fluoropolymer is positionedon an external surface of the first tubular member and on an interiorsurface of the second tubular member. The serpentine fibrils have awidth of about 1.0 micron or less. In at least one embodiment, thefluoropolymer is fluorinated ethylene propylene.

It is a further object of the present invention to provide a method offorming a covered stent that includes (1) forming a tube having acomposite material that includes an expanded polytetrafluoroethylenemembrane and a discontinuous coating of a fluoropolymer thereon, wherethe fluoropolymer is positioned on an external surface of the tube, (2)cutting the tube cross sectionally to form a first tubular member and asecond tubular member, (3) everting the second tubular member toposition the fluoropolymer on an interior surface of the second tubularmember, (4) positioning the first tubular member within a stent, (5)positioning the second tubular member on an external surface of thestent, and (6) heating the stent having thereon the first and secondtubular members to adhere the fluoropolymer on the first tubular memberto the second tubular member through interstices of the stent to form acovered stent. In exemplary embodiments, the expanded fluoropolymermembrane includes serpentine fibrils. The serpentine fibrils have awidth of about 1.0 micron or less.

The foregoing and other objects, features, and advantages of theinvention will appear more fully hereinafter from a consideration of thedetailed description that follows. It is, to be expressly understood,however, that the drawings are for illustrative purposes and are not tobe construed as defining the limits of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The advantages of this invention will be apparent upon consideration ofthe following detailed disclosure of the invention, especially whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of an exemplary, idealized serpentinefibril;

FIG. 2 is a scanning electron micrograph (SEM) of a retracted membranewith a discontinuous coating of FEP taken at 200×;

FIG. 3 is a scanning electron micrograph (SEM) of the surface of a stentcover that includes an expanded fluoropolymer membrane having adiscontinuous coating of fluoropolymer thereon where the expandedfluoropolymer membrane includes serpentine fibrils that have a firstcurve in a first direction, a second curve in a second direction, and athird curve in a third direction taken at 10,000×; and

FIG. 4 is a graphical illustration of a pressure vs. diameter curve ofan exemplary stent graft according to the present invention where theintersection of the tangent lines depicts the stop point of thecomposite material.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention belongs. In the drawings, the thicknessof the lines, layers, and regions may be exaggerated for clarity. Likenumbers found throughout the figures denote like elements.

The present invention is directed to fluoropolymer membranes thatexhibit high elongation while substantially retaining the strengthproperties of the fluoropolymer membrane. Such membranescharacteristically possess serpentine fibrils, such as the idealizedserpentine fibril exemplified in FIG. 1. As depicted generally in FIG.1, a serpentine fibril curves or turns generally one way in thedirection of arrow 10 then generally another way in the direction ofarrow 20. It is to be understood that the amplitude, frequency, orperiodicity of the serpentine-like fibrils as exemplified in FIG. 1 mayvary. In one embodiment, the fluoropolymer membranes are expandedfluoropolymer membranes. Non-limiting examples of expandablefluoropolymers include, but are not limited to, expanded PTFE, expandedmodified PTFE, and expanded copolymers of PTFE. Patents have been filedon expandable blends of PTFE, expandable modified PTFE, and expandedcopolymers of PTFE, such as, for example, U.S. Pat. No. 5,708,044 toBranca; U.S. Pat. No. 6,541,589 to Baillie; U.S. Pat. No. 7,531,611 toSabol et al; U.S. patent application Ser. No. 11/906,877 to Ford; andU.S. patent application Ser. No. 12/410,050 to Xu et at.

The high elongation is enabled by forming relatively straight fibrilsinto serpentine fibrils that substantially straighten upon theapplication of a force in a direction opposite to the compresseddirection. The creation of the serpentine fibrils can be achievedthrough a thermally-induced controlled retraction of the expandedpolytetrafluoroethylene, through wetting the article with a solvent(followed by drying), or by a combination of these two techniques. Thesolvent may be, but is not limited to, isopropyl alcohol or Fluorinert®(a perfluorinated solvent commercially available from 3M, Inc., St.Paul, Minn.). In general, for unrestrained articles, the higher thetemperature and the longer the dwell time, the higher the degree ofretraction up to the point of maximum retraction. In addition, the speedof retraction can be increased by increasing the retraction temperature.The retraction of the membrane does not result in visible pleating,folding, or wrinkling of the ePTFE, unlike what occurs during mechanicalcompression. The retraction also can be applied to very thin membranes,unlike known methods. During the retraction process, the fibrils becomeserpentine in shape and, in some instances, may also increase in width.

The precursor materials can be biaxially expanded ePTFE membranes. Inone embodiment, materials such as those made in accordance with thegeneral teachings of U.S. Pat. No. 7,306,729 to Bacino, et al. aresuitable precursor membranes, especially if small pore size articles aredesired. These membranes may possess a microstructure of substantiallyonly fibrils. The precursor membrane may or may not be amorphouslylocked. Additionally, the precursor membrane may be at least partiallyfilled, coated, imbibed, or otherwise combined with additionalmaterials. For example, the precursor membrane may contain or be atleast partially coated or imbibed with a fluoropolymer, such as, forexample, fluorinated ethylene propylene.

The precursor membrane may be restrained in one or more directionsduring the retraction process in order to prescribe the desired amountof elongation of the final article. The amount of elongation is directlyrelated to, and is determined by, the amount of retraction. In theinstant invention, the amount of retraction can be less than about 90%,75%, 50%, or 25% of the initial un-retracted length. The resultantamounts of elongation in the direction of retraction can be at leastabout 60%, 80%, 100%, 200%, 300%, 400%, 500%, 600%, or even greater,including any and all percentages therebetween.

The retraction temperature range includes temperatures that result inthe retraction of the precursor membrane. In some instances, theretraction temperature can exceed the amorphous locking temperature ofthe precursor membrane.

In one embodiment, retraction can be achieved in a uniaxial tenter frameby positioning the rails at a distance less than the width of theprecursor membrane prior or during the application of heat or solvent orboth. When using a biaxial tenter frame, one or both of the sets ofgrips, pins, or other suitable attachment means can similarly bepositioned at a distance less than the dimensions of the precursormembrane. It is to be appreciated that these retraction means differfrom the mechanical compression taught by the House and Sowinski patentsnoted above.

In another embodiment, the article can be retracted while being held byhand. A tubular article can be retracted by fitting it over a mandrelprior to retraction. In yet another embodiment, the membrane can beplaced in an oven and allowed to retract unrestrained. It is to beunderstood that any suitable means of retracting the article that doesnot result in the formation of visible folds, pleats, or wrinkles can beemployed.

The resulting retracted articles surprisingly exhibit high elongationwhile substantially retaining the strength properties of thefluoropolymer membrane. Upon retraction, the expanded fluoropolymermembrane possesses serpentine fibrils. These retracted membranescharacteristically possess serpentine fibrils and are free of wrinkles.In some exemplary embodiments, the retracted membranes may possess amicrostructure of substantially only serpentine fibrils. In certaininstances, it may be necessary to partially elongate the retractedmembrane in order to observe the serpentine fibrils with magnification.In at least one embodiment, the fluoropolymer membranes include aplurality of serpentine fibrils. As used herein, the phrase “pluralityof serpentine fibrils” is meant to denote the presence of 2 or more, 5or more, 10 or more, or 15 or more serpentine fibrils in thefluoropolymer membrane within a field of view as taught below. Theserpentine fibrils have a width of about 1.0 micron or less, and in someembodiments, about 0.5 microns or less. In one embodiment, theserpentine fibrils have a width from about 0.1 to about 1.0 microns, orfrom about 0.1 to about 0.5 microns.

In another embodiment of the present invention, the precursor membranesdescribed above can be imbibed with an elastomeric material prior,during, or subsequent to retraction to form a composite material. In theabsence of such elastomeric materials, fluoropolymer articles havingserpentine fibrils do not exhibit appreciable recovery after elongation.Suitable elastomeric materials include, but are not limited to, PMVE-TFE(perfluoromethylvinyl ether-tetrafluoroethylene) copolymers, PAVE-TFE(perfluoro (alkyl vinyl ether)-tetrafluoroethylene) copolymers,silicones, polyurethanes, and the like. It is to be noted that PMVE-TFEand PAVE-TFE are fluoroelastomers. Other fluoroelastomers are suitableelastomeric materials. The resultant retracted article not onlypossesses high elongation while substantially retaining the strengthproperties of the fluoropolymer membrane, but also possesses theadditional property of low percent unrecoverable strain energy density.These articles can exhibit percent unrecoverable strain energy densityvalues less than about 85%, less than about 80%, less than about 70%,less than about 60%, and lower, including any and all percentagestherebetween.

In another embodiment of the invention, the precursor membrane isimbibed or coated, at least partially or completely, or otherwisecombined with at least one other material that may include, but is notlimited to, fluorinated ethylene propylene (FEP), other fluoropolymers,polymers, copolymers, or terpolymers, ethylene fluorinated ethylenepropylene (EFEP), THV (a terpolymer of tetrafluoroethylene,hexafluoropropylene, and vinylidene fluoride), PFA (perfluoroalkoxycopolymer resin), ECTFE (ethylene chlorotrifluoroethylene), PVDF(polyvinylidene fluoride), and PEEK (polyether ether ketone). Thefluoropolymer membrane may be imbibed or coated with this other materialduring, prior, or subsequent to retraction. The fluoropolymer (or othermaterial) may also or alternatively be located in at least a portion ofor all of the pores of the fluoropolymer membrane.

A further embodiment of the invention takes advantage of a beneficialproperty of the inventive composite material (i.e., an expandedfluoropolymer membrane having a discontinuous coating of a fluoropolymerthereon). Composite materials of this invention not only exhibitelongation, but also exhibit a dramatic increase in stiffness afterachieving a high, optionally predetermined, elongation. As aconsequence, the composite materials can be elongated to a point atwhich further elongation is inhibited by the dramatic increase instiffness. That is, the composite material has a stop point at whichfurther expansion, elongation, or both occur only in conjunction with asignificant increase in pressure or force. Additionally, the compositematerial is substantially free of wrinkles.

In one specific instance, the inventive composite material can be usedto create a covered stent device that requires little pressure todistend to a first diameter but which is highly resistant to furtherdistension after reaching a certain diameter. The result is that thedevice can be distended in the radial direction under relatively lowforce until reaching that certain diameter. This diameter is a functionof the inventive composite material. In other words, the diametricincrease in the diameter of the covered stent device prior to reachingthe stop point is a function of the inflection point in the elongationversus force curve for the inventive material, which in turn, is afunction of the degree of retraction of the precursor membrane. Abenefit of the composite material is that a large increase in diameterof the covered stent device can be achieved prior to reaching the stoppoint. The stop point of the composite material in exemplary coveredstents may occur at a diameter of at least about 7 mm, at least about 8mm, at least about 9 mm, at least about 10 mm, or even greater. Onesignificance of the stop point is that the stent graft does not itselfbecome aneurismal.

When the composite material is used as a cover for a stent, thecomposite material remains wrinkle-free and fold-free regardless of thediameter of the covered stent device. For purposes of this invention,the entire device is considered to be “wrinkle-free” if within a 1 cmlength of the device, the graft portion is devoid of wrinkles and foldswhen viewed by the naked eye. It is to be noted that 1 cm length of thedevice should be used unless the entire length of the device is lessthan 1 cm. In that instance, the entire device should be utilized todetermine if the device is “wrinkle-free.” The ability of the cover toremain wrinkle-free results in less or no material infolding duringcompaction, which, in turn, permits the resulting covered stent deviceto have a smaller profile (e.g., a reduction in delivery profile of atleast about 1 Fr). The absence of folds in the cover reduces oreliminates the potential for thrombus accumulation that can ultimatelyresult in total occlusion of the device. Also, the composite materialexhibits high elongation until reaching a length elongationcorresponding to an increase in stiffness while substantially retainingthe strength properties of the fluoropolymer membrane. The extension ofthe serpentine fibrils in the composite material to a substantiallystraight orientation surprisingly retains the strength properties of thefluoropolymer membrane. The composite material allows the cover to beattached to the stent at a small stent diameter and when the stent graftis expanded, the cover does not develop folds. In addition, thecomposite material both inhibits over-distension of the stent graft andallows over-distension with the application of a substantially higherforce. Additionally, the covered stent exhibits minimal foreshorteningduring the expansion process prior to over-distension.

It is to be appreciated that the break strength of the cover can bealtered by using few or numerous layers of the composite material tocover the stent. Alternatively, or in addition to, weaker or strongerfluoropolymer membranes can be used to achieve the same or substantiallythe same effect.

It is also to be appreciated that when the composite material is used asa cover for a stent, the fluoropolymer coating on the expandedfluoropolymer membrane is not substantially attached to the stent. Asused herein, the term “substantially attached” means that thefluoropolymer is not attached to the stent or is only minimally attachedto the stent. Rather, the fluoropolymer is utilized as an adhesive toaffix two composite materials together to form the cover. For example, afirst tube having a fluoropolymer-covered exterior surface and a secondtube having a fluoropolymer interior surface are positioned on theinterior and exterior surfaces of a stent, respectively, such that thefluoropolymer on the first tube adheres to the fluoropolymer on thesecond tube (i.e., the opposite surface) through the interstices of thestent, thereby creating a covered stent device. In the presentinvention, it is important that the composite material forming the coveris not firmly attached to the stent. If the composite material wasfirmly attached, the cover would tear upon expansion of the coveredstent device. That is, the primary means of attachment is achieved bybonding the fluoropolymer FEP) portions of the composite covers throughthe interstices of the stent.

Articles of the present invention can take various forms including, butnot limited to, sheets, tubes, covers, and laminates.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples illustrated belowwhich are provided for purposes of illustration only and are notintended to be all inclusive or limiting unless otherwise specified.

Testing Methods

It should be understood that although certain methods and equipment aredescribed below, any method or equipment determined suitable by one ofordinary skill in the art may be alternatively utilized.

Thickness

Membrane samples were die cut to form rectangular sections about 2.54 cmby about 15.24 cm to measure thickness (using a Käfer Fz1000/30 snapgauge). The average of three measurements was reported.

Scanning Electron Microscopy

Scanning electron micrographs were created choosing magnificationssuitable for identifying fibrils. Articles that have been retracted inaccordance with the teachings of invention may require elongation in thedirection of retraction in order to identify the serpentine fibrils. Forthe purposes of identifying the number of serpentine fibrils, a field ofview of 7 microns by 7 microns of the sample is to be employed.

In addition, for the purpose of characterizing fibril width,measurements should be made for serpentine fibrils that aresubstantially separated from each other and do not band together orotherwise form series of fibrils paralleling each other within themembrane. To determine the fibril width, a line is drawn through the SEMimage to bisect it. The SEM image should be of sufficient magnificationsuch that at least 5 serpentine fibrils and not more than 20 serpentinefibrils are clearly visible within the SEM image. Starting from one edgeof the bisected image, the width of the first five consecutiveserpentine fibrils that intersect the bisecting line are measured. Themeasurements are made where the fibril intersects the bisecting line.Next, the five measurements are averaged and the average measurement isrecorded.

Radial Elongation Test

The test method that follows describes the test method for an 8 mmcovered stent:

A covered stent that had been crushed to its delivery diameter onto anas-packaged (i.e., deflated) 8 mm balloon was positioned on the end of aballoon catheter. The covered stent was placed within the measuring zoneof a laser micrometer (e.g., DataMike Model 700, TechMet Co., Dayton,Ohio).

A balloon inflator (e.g., COMPAK balloon inflator, Merit Medical, SouthJordan, Utah) was obtained. The balloon inflator was filed with waterand attached to the luer fitting of the balloon catheter.

The handle on the inflator was slowly turned while the change inpressure as indicated on the dial was observed. The balloon was theninflated to 2 atmospheres of pressure. The stent easily continued toexpand, causing the pressure to drop. The inflation was continued untilthe pressure of 2 ATM was maintained. The stent was then inflated at 1ATM intervals and the diameters at those pressures once pressureequilibrated were recorded. Inflation was continued until 14 atmosphereswas achieved, which was 1 ATM below the rated burst pressure of theballoon.

A similar procedure should be followed for different sizes of coveredstents. For other covered stent sizes, choose an appropriately sizedballoon. Continue inflating until reaching 1 ATM below the rated burstpressure of the balloon.

The pressure-diameter curves relating to composite materials and coveredstents of the present invention exhibit an inflection point due to thechange in slope upon reaching a diameter referred to herein as the stoppoint. FIG. 4 is a graphical illustration of a pressure vs. diametercurve of an exemplary stent graft according to the present inventionwhere the intersection of the tangent lines depicts the stop point ofthe stent graft. The intersection of the tangent lines is depicted byreference numeral 70. An estimate of the stop point may be determined inthe following manner. The slope of the pressure-diameter curve prior toreaching the stop point can be approximated by drawing a straight linetangent to the curve as shown as line 50 in FIG. 4. The slope of thepressure-diameter curve beyond the stop point can be approximated bydrawing a straight line tangent to the curve as shown as line 60 in FIG.4. The diameter corresponding to the intersection of the two tangentlines is an estimation of the stop point for that composite material.

EXAMPLES Example 1

Expanded Fluoropolymer Membrane with Discontinuous FEP

Fine powder of PTFE polymer as described and taught in U.S. Pat. No.6,541,589 was blended with Isopar K (Exxon Mobil Corp., Fairfax, Va.) inthe proportion of 0.209 g/g of fine powder. The lubricated powder wascompressed into a cylinder to form two pellets that were placed into anoven set at 49° C. for approximately 12 hours. The compressed and heatedpellets were ram extruded to produce tape approximately 16.2 cm wide by0.70 mm thick. The two extruded tapes were then layered and rolled downbetween compression rolls to a thickness of 0.381 mm. The calendaredtape was then transversely stretched to 32 cm (i.e., at a ratio of2.0:1) and dried at a temperature of approximately 230° C. Anapproximately 12.5 um thick by approximately 28 cm wide FEP filmavailable from Dupont De Numerous, Inc., (Wilmington, Del.) wasobtained. The calendered PTFE tape and the FEP film were laminatedtogether during a longitudinal expansion process that consisted ofstretching the two materials, in contact with one another, between banksof rolls over a heated plate set to a temperature of 300° C. The speedratio between the second bank of rolls to the first bank of rolls was10:1. The width of the resulting longitudinal expanded membrane wasapproximately 14 cm. The longitudinally expanded membrane (having FEPfilm laminated on one side) was then expanded transversely at atemperature of approximately 280° C. to a ratio of about 30:1 and thenconstrained from shrinkage and heated in an oven set at 360° C. forapproximately 10 seconds. The resulting expanded fluoropolymer membranehad regions of FEP discontinuously located on one surface thereof.

This expanded, discontinuously coated fluoropolymer membrane wasthermally retracted in the following manner. A roll of precursormembrane where the length direction corresponded with the strongestdirection of the membrane, was restrained in the clamps of a heated,uniaxial tenter frame and fed into the heated chamber of the tenterframe. The oven temperature was set to about 270° C. The rails of thetenter frame within the heated chamber were angled inward to allowmembrane shrinkage to about 24.6% of its original width in response tothe heat. The membrane was retracted over a period of time ofapproximately 20 seconds.

A scanning electron micrograph of the retracted membrane is provided inFIG. 2, in which the magnification was 200×. Note the presence ofregions of FEP, depicted by reference numeral 40, discontinuouslylocated on the surface.

Example 2

The retracted membrane of Example 1 was used to create a covered stentdevice. An 8 mm diameter×6 mm long stainless steel stent (CordisPalmaz-Schatz Transhepatic Biliary stent, Cat. No. PS5608A, Lot No.80599853, Cordis Corp., Bridgewater, N.J.) was obtained. The retractedmembrane of Example 1 was used to cover the stent as follows. A tube wasconstructed from a 150 mm wide sample of the membrane. A 4 mm diameter,150 mm long stainless steel mandrel was obtained. Twelve layers of the150 mm wide membrane were circumferentially wrapped around the mandrelsuch that the retracted direction of the membrane was oriented along thecircumferential axis of the mandrel. The FEP side of the membrane facedoutward. A soldering iron set to approximately 320° C. was used to spottack the free edge of the film. A 1.3 cm wide slit of an ePTFE film waswrapped on each end of the tube to avoid longitudinal retraction duringsubsequent heating. The assembly was then placed into an oven set to340° C. for about 20 minutes, thereby creating a tube. The tube wasallowed to cool and was removed from the mandrel. The tube was cut intotwo 75 mm lengths. One length was everted in order to position the FEPon the interior surface of the tube.

The tube with the FEP positioned on the exterior surface was placed overthe 4 mm stainless steel mandrel. The 8 mm stent was partially inflatedto about 3 mm (using the balloon onto which the commercially availablestent had been mounted). The stent was removed from the balloon andslipped onto a tapered 4.5 mm stainless steel mandrel in order toincrease its diameter. The stent was removed from the 4.5 mm mandrel andslipped on top of the membrane-covered 4 mm stainless steel mandrel. Theeverted tube was then placed over top of the stent. A 5 mm innerdiameter, 0.75 mm thick extruded, expanded sacrificial ePTFE tube wasplaced over the tube/stent assembly. An iris-style radial crushingdevice (Blockvvise Engineering LLC, Tempe, Ariz.) was used to bring theexterior tube through the stent openings into contact with the innertube, thereby bringing the FEP on both tubes into contact.

With the tubes still in contact, an ePTFE film was wrapped around theoutside of the exterior sacrificial tube. The entire assembly was thenplaced, in an oven set to 320° C. for about 15 minutes. The assembly wasremoved from the oven, allowed to cool, and the now-formed covered stentwas removed from the mandrel and sacrificial layers. The excess tubematerial at the ends of the stent was trimmed. The covered stent wascrushed onto the deflated 8 mm balloon on which the stent had beenprovided.

A scanning electron micrograph of the cover inflated to approximately 4mm is provided in FIG. 3 at a magnification of 10,000×.

A balloon inflator (COMPAK balloon inflator, Merit Medical, SouthJordan, Utah)) was obtained and utilized to create a pressure-diametercurve as shown in FIG. 4 using the method described above. FIG. 4 is thepressure-diameter curve corresponding to the covered stent where thecover is formed of the retracted membrane containing discontinuous FEP.As shown in FIG. 4, the retracted membrane can be elongated at a lowpressure until reaching the diameter where the slope of the curvesubstantially decreases, indicating an increased stiffness. The stentcover remained wrinkle-free throughout the test. At about 3 ATM, thestent began to expand. Once about 9 ATM was reached, the stent resistedfurther expansion due to the presence of the cover. The covered stentexhibited minimal foreshortening during the expansion process and thecomposite material exhibited a stop point at a diameter of about 7 mm.

The invention of this application has been described above bothgenerically and with regard to specific embodiments. The invention isnot otherwise limited, except for the recitation of the claims set forthbelow.

What is claimed is:
 1. A method of forming a covered stent comprising:positioning a first wrinkle-free tubular member on an interior surfaceof a stent; positioning a second wrinkle-free tubular member on anexternal surface of said stent, each said tubular member comprising acomposite material including an expanded polytetrafluoroethylenemembrane having serpentine fibrils and a discontinuous coating of afluoropolymer thereon, said fluoropolymer being positioned on anexternal surface of said first and tubular member and on an interiorsurface of said second tubular member; and heating said stent havingthereon said first and second tubular members to adhere saidfluoropolymer on said first tubular member to said second tubular memberthrough interstices of said stent and form a covered stent.
 2. Themethod of claim 1, wherein the fluoropolymer is fluorinated ethylenepropylene.
 3. The method of claim 1, wherein the expanded fluoropolymermembrane comprises a microstructure of substantially only serpentinefibrils.
 4. The method of claim 1, wherein the expanded fluoropolymermembrane comprises a plurality of serpentine fibrils.
 5. The method ofclaim 1, wherein said composite material is radially expanded to adiameter beyond which further expansion is inhibited.
 6. The method ofclaim 1, wherein the expanded fluoropolymer membrane comprises pores andthe fluoropolymer at least partially penetrates a plurality of saidpores.
 7. The method of claim 1, wherein said composite materialexhibits an increase in stiffness when expanded to a diameter of about 7mm.
 8. The method of claim 1, wherein each said serpentine fibril has awidth of about 1.0 micron or less.
 9. The method of claim 1, whereineach said serpentine fibril has a width of 0.5 microns or less.